How do I make the light go behind the Earth?
The lens flare you refer to as "Light" should according to normal physics never be blocked by an obstacle. If you still want this effect tho, you should try to mask out the earth in your post processing. To do this, create a second layer in which you render your earth completely white with your background completely black. Do this, with either the emission shader or an background shader. You also need to render a layer without lens flare. Then you can use your mask and to overlay the flare less render above the one with flare using an alpha over node.